The typical epicyclic gear system has a sun gear, a ring gear surrounding the sun gear, and several, often three, planet pinions located between and engaged with the sun and ring gears, and in addition, it has a carrier that is coupled to the planet pinions to establish axes about which they rotate. A gear system so configured splits the torque transferred through the system into load paths equal in number to the number of planet pinions. This reduces the forces at each mesh and enables the system to transfer a large amount of power in a relatively compact configuration. In other words, it provides a high power density.
Often the ring gear remains fixed, leaving the carrier and sun gear to rotate. In such an arrangement power may be applied at one end of the carrier and delivered through the sun gear at a different velocity and torque. This holds true for the transmissions in wind turbines that harness the energy of the wind and convert it into electrical power.
Typically, epicyclic gear systems rely on spur gearing in which the teeth of the gears lie parallel to the axes of the gears. However, helical and double helical gears are available, but have not found favor.
Irrespective of the type of gearing, machining tolerances in the manufacture of such gears leave variations in the meshes between the several planet pinions and the sun and ring gears. As torque is applied to the system the mesh with the least clearance transfers the load by itself, until that mesh deflects enough to enable the mesh with the next least clearance to take some of the load. The progression continues until all of the meshes accommodate the load. But some of the meshes support more of the load than others.
Many epicyclic gear systems utilize a straddle-type carrier in which the planet pinions rotate between two walls on pins that extend between the walls, each pin being anchored at both of its ends in the walls. When torque is applied to the carrier at one of the end walls, the carrier will twist and advance one end of each pin ahead of the other. This skews the planet pinions with respect to the sun and ring gears, and disturbs the mesh between the planet pinions and the sun and ring gears.
An epicyclic gear system in which the planet pinions are supported on and rotate about so-called “flexpins” helps mitigate unequal load distribution among the planet pinions and skewing of the pinions as well. In this regard, a flexpin for a planet pinion at one end is anchored in and cantilevered from the wall of the carrier of which it is a part. The other end of the flexpin has a sleeve fitted firmly to it, with the sleeve extending back over and otherwise spaced radially from the flexpin. The sleeve forms part of or carries a bearing that supports one of the planet pinions. At the carrier wall the flexpin bends in one direction circumferentially relative the main axis and at its opposite end bends in the other direction, again circumferentially, all such that the sleeve remains parallel to the axis. In other words, flexpin technology employs a double cantilever to equalize load distribution and to offset the skewing that would otherwise occur. See U.S. Pat. No. 7,297,086 and WO 2007/016336, which are incorporated herein by reference, for a further discussion of flexpin technology.
Helical gears operate more smoothly and with less noise than spur gears. Moreover, the teeth on a helical gear are longer and stronger than the teeth on a spur gear of equivalent size. But helical gearing has not found favor in epicyclic gear systems that utilize flexpin technology, because the planet pinions experience thrust loading at the mesh between the sun gear and planet pinions and additional thrust loading at the mesh between the planet pinions and ring gear—indeed, in the opposite directions. As a result, each planet pinion undergoes a couple that tilts the sleeve about which it rotates toward or away from the main axis of the system. While flexure of the flexpins circumferentially along the pitch circle for the carrier serves to improve the mesh between the planet pinions and the sun and ring gears, flexure toward and away from the main axis disturbs the mesh, increasing wear and noise. To be sure, herringbone gears with double helices eliminate the problem, but the manufacture of such gears introduces complexities and expense not present with helical gears.